Semiconductor device

ABSTRACT

A semiconductor device to output voltages at three levels to a word driver while alleviating the breakdown voltage in the MOS transistor. This invention is comprised of a breakdown-voltage reducing MOS transistor inserted in the word driver and two NMOS transistors to supply a read-out voltage to a word line. The word driver is moreover controlled by different voltage amplitudes on the main word lines and the common word lines.

This is a continuation of application Ser. No. 10/201,317 filed 24 Jul.2002, now U.S. Pat. No. 6,611,474 which is division of application Ser.No. 09/706,374 filed 3 Nov. 2000 now U.S. Pat. No. 6,452,858, thecontents of which are incorporated herein by reference in theirentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a semiconductor device and relates inparticular to a semiconductor memory device. More particularly, thisinvention relates to a semiconductor memory device containing a highlyintegrated and highly reliable memory utilizing an amplifying memorycell.

2. Description of Related Art

The widely used dynamic random access memory (DRAM) is a singletransistor cell utilized as a memory cell and consisting of a singletransistor and a single capacitor. However in recent years, as MOStransistors (MOSFET: Metal Oxide Semiconductor Field Effect Transistor)in semiconductor devices become more highly integrated and moreminiaturized, the breakdown voltage becomes lower and the operatingvoltage has also become lower to achieve lower electrical powerconsumption also becomes lower. In addition, in a DRAM utilizing asingle transistor cell, the memory cell itself has no amplifying actionso that the read out signal level from the memory cell is small andoperation tends to be unstable because of effects from all types ofnoise.

So, a memory cell utilizing three transistors (hereafterthree-transistor cell) and previously used prior to the singletransistor cell is again attracting attention as a memory cell capableof delivering a large read-out signal level by an amplifying action.This three-transistor cell is described for instance in the IEEEInternational Solid-State Conference, DIGEST OF TECHNICAL PAPERS, pp.10-11, 1972).

This memory cell for example as shown in FIG. 2, is comprised of aread-out NMOS transistor QR, a write NMOS transistor QW, and also acharge holding NMOS transistor QN. The gates of the transistors QR andQW are connected to the word line WL, and the source is connected to thedata line DL. The gate of the transistor QN is connected to the drain ofthe transistor QW, and the source of the transistor QN is connected tothe source line SL. The transistor QN, QR drains are also connected.Here, the threshold voltage VTW of the transistor QW is set higher thanthe threshold voltage VTR of transistor QR, and the data line voltageamplitude is equal to the supply voltage amplitude VDL. In a memory cellconfigured this way, the word line voltage for the write operation mustbe a high write voltage VW higher than the threshold voltage VTW, andthis value is generally set higher than the supply voltage VDL. Also,the word line voltage for the read operation must be a read voltage VRhigher than the threshold voltage VTR, and lower than the thresholdvoltage VTW and this value is generally set between the supply voltagelevel VDL and ground potential. Further, the standby state (non-selectstate) of the word line voltage must be lower than the word line voltageVTR and is set for example at ground potential VSS.

A device having an amplifying memory cell comprised of one capacitor andtwo transistors (hereafter called capacitive coupling 2-transistor cell)is described in IEEE ELECTRONICS LETTERS 13th May, 1999 Vol. 35 No. 10,pp. 848-850).

This memory cell as shown in FIG. 3, is comprised of a read NMOStransistor QR, a write transistor QW, and also a coupling capacitor Ccfor controlling the voltage of the memory cell node N. The transistorsQR and QW are in a stacked configuration so this device is characterizedby a small surface area. A transistor utilizing the tunnel effect isused as transistor QW so the leak current is small. These components areconnected as follows. One end of the capacitor Cc and the gate oftransistor QW are connected to the word line WL, and the source oftransistor QW is connected to bit line BL. The other end of thecapacitor Cc and the drain of the transistor QW are connected to thegate of the transistor QR, and the memory cell node N thus formed. Thesource of the transistor QR is grounded, and the drain connected to thesense line SL. The word line voltage VW for writing and the word linevoltage VR for reading are respectively set in this kind of cell, asdescribed for the three-transistor cell shown in FIG. 2.

However, in the standby state (non-select state), the voltage potentialVN (H) for the standby state of the memory cell node N written at thesupply voltage level VDL, must be a word line voltage at a lower voltagepotential than VTR, for instance the standby voltage −VB must be setlower than the ground voltage VSS. Therefore, in the three-transistorcell and the capacitive coupling type 2-transistor cell as describedabove, the read and write operation is controlled by a read voltage VRand write voltage VW applied to one word line.

SUMMARY OF THE INVENTION

This invention therefore has the object of achieving a high speed, lowcurrent consumption, high integration DRAM for maintaining highreliability. This invention also has the object of providing asemiconductor device containing a highly integrated and highly reliablememory utilizing an amplifying memory cell.

More specifically, this invention is two aspects as described next. Thefirst aspect is a sub-word driver to drive a sub-word line with a3-value word line voltage, and also a DRAM utilizing this word driver.The second aspect is a high speed, low current consumption, highintegration DRAM maintaining high reliability and eliminating theproblem of breakdown voltage in MOS transistors with this sub-worddriver.

Hereafter, the background of this invention is related in detail whilereferring to example of the prior art.

Along with the higher integration and lower power consumption of DRAMdevices, the delay time in the word line has become a problem. As onemeans to resolve the delay time problem, a hierarchical word linestructure to divide these word lines in order to reduce their capacitiveload, drive each line with separate drivers installed on each line, andhaving drivers installed on the each of the divided word line WL hasbeen proposed. A sub-word driver utilizing such a structure has beendescribed in the European Solid-State Circuits Conference Digest ofTechnical Papers, pp 131-134, September 1992.

The circuit structure is shown in FIG. 4. The circuit structure SWDenclosed by dashed lines in FIG. 4 is the area of the sub-word driver. Amain word line MWLb is connected to the gates of the PMOS transistor Mp1and the NMOS transistor Mn1. A common word line FXb is connected to thegate of the NMOS transistor Mn2. A common word line FXt is connected tothe source of the transistor Mp1, and the sources of the transistors Mn1and Mn2 are grounded. The main word lines from the drains of thetransistors Mp1, Mn1, Mn2 connect to the branching sub-word line SWL.

The operation of the circuit of FIG. 4 is next described by referring toFIG. 5. When the main, word line MWLb at the high level supply voltageVDL is driven to a low level, ground level VSS, the common word line FXtat ground potential VSS is driven to supply voltage level VDL so that asshown in FIG. 4, the transistor Mp1 for the sub-word driver conductsand, the sub-word line SWL at ground potential VSS is driven to selectstatus at supply voltage VDL. In this way, the voltage level of thesub-word line SWL of the prior art sub-word driver is driven to one oftwo levels: a high level or a low level.

As related above, a memory array using a three-transistor cell orcapacitive coupling 2-transistor cell having low voltage operation, mustset the word line to three values. Therefore, a sub-word driver capableof driving the sub-word line to voltage levels of three values isrequired in order to use this hierarchical word line structure. The gateoxidation film of the MOS transistors in the peripheral circuits shouldpreferably be made thin to prevent a drop in MOS transistor performanceeven during low voltage operation. Due to these factors, the maximumpermissible electric field of the oxidation film of the MOS transistorsin the applicable peripheral circuits therefore tends to drop.

However, when a MOS transistor having the same oxidation film thickness(tox), as the peripheral transistors is used in sub-word drivers, thesub-word line voltage amplitude for the three values required for thecapacitive coupling 2-transistor cell, as related above, is larger thanthe supply voltage amplitude so that the MOS transistor breakdownvoltage problem is unavoidable.

This invention resolves the above described problems.

In a typical example of the invention to achieve the above objects, asemiconductor device has a plurality of word lines, a plurality of datalines intersecting with the plurality of word lines, a plurality ofmemory cells installed at desired cross points of the plurality of datalines and plurality of words lines, and a plurality of word drivers todrive the plurality of word lines, wherein each of the plurality of worddrivers has a first conduction first MOS transistor supplied with afirst voltage to either the source or the drain, a second conductionfirst MOS transistor supplied with a second voltage to either the sourceor the drain, a second conduction second MOS transistor supplied with asecond voltage to either the source or the drain for at least thedesired time, a second conduction third MOS transistor supplied with athird voltage to either the source or drain, a second conduction fourthMOS transistor connected at either the source or the drain to the othersource or drain of the second conduction third MOS transistor, and eachof the plurality of word drivers outputs any one of the first voltage,the second voltage or third voltage.

The “MOS transistor or MOSFET” of these specifications are abbreviatedterms signifying an insulated gate, metal-oxide-semiconductorfield-effect-transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the sub-word driver for generating thethird value voltage level.

FIG. 2 is a drawing showing an example of the memory cell comprised ofthree transistors.

FIG. 3 is a drawing showing an example of the memory cell comprised oftwo transistors and one capacitor.

FIG. 4 is a drawing showing the circuit of the sub-word driver of therelated art.

FIG. 5 is a drawing showing the operation timing of the sub-word driverof the related art.

FIG. 6 is a drawing showing typical preferred voltage settings in thecapacitive coupling type 2-transistor cell DRAM.

FIG. 7 is a circuit diagram showing a portion of the hierarchical wordline structure of the first embodiment.

FIG. 8 is a drawing showing operating timing diagrams of sub-worddrivers generating three value voltage levels.

FIG. 9 is a drawing showing the circuit diagram of the main word driverof the first embodiment.

FIG. 10 is a circuit diagram of the common word driver circuit.

FIG. 11 is a drawing showing a typical memory cell array.

FIG. 12 is operation timing diagrams of the memory cell comprised of twotransistors and one capacitor.

FIG. 13 is a circuit diagram of the sub-word driver for generating thethree value voltage levels of the second embodiment.

FIG. 14 is a circuit diagram for the common word driver of the secondembodiment.

FIG. 15 is operation timing diagrams of the sub-word driver forgenerating the three value voltage levels of the second embodiment.

FIG. 16 shows operation timing diagrams of the sub-word driver forgenerating the three value voltage levels of the third embodiment.

FIG. 17 shows operation timing diagrams of the sub-word driver forgenerating the three value voltage levels of the third embodiment.

FIG. 18 is a circuit diagram of the main word driver of the thirdembodiment.

FIG. 19 is a circuit diagram of the common word driver of the thirdembodiment.

FIG. 20 is a circuit diagram for the sub-word driver for generating thethree value voltage levels of the fourth embodiment.

FIG. 21 shows operation timing diagrams of the sub-word driver forgenerating the three value voltage levels of the fourth embodiment.

FIG. 22 is a circuit diagram of the main word driver of the fourthembodiment.

FIG. 23 is a circuit diagram of the common word driver of the fourthembodiment.

DETAILED DESCRIPTION

First, an overall description of the embodiments of the invention isgiven and then working examples of those embodiments are explained indetail. A semiconductor device utilizing a hierarchical word linestructure is comprised of a plurality of sub-word lines, a plurality ofdata lines installed to intersect with the plurality of sub-word lines,a main word line installed roughly in parallel with the plurality ofsub-word lines, a plurality of common word lines installed to intersectwith the plurality of sub-word lines, a plurality of memory cells forexchanging signals with the data lines selected by word lines installedat the desired cross points of the plurality of data lines and pluralityof sub-word lines, a plurality of sub-word drivers to drive each of theplurality of sub-word lines selected by the applicable common word lineand main word line installed at the desired cross point of the pluralityof common word lines and plurality of main word lines, a read circuit toamplify the signal from the memory cell set according to the pluralityof data lines, and a write circuit to write the signal from the memorycell set according to the plurality of data lines, wherein each of theplurality of sub-word drivers generates a first word line voltage forthe write operation, each of the plurality of sub-word drivers generatesa second word line voltage for the standby state, each of the pluralityof sub-word drivers generates a third word line voltage for the readoperation, and the semiconductor device is further configured so thatthe voltage applied to the gate oxidation film of the MOS transistorscomprising the plurality of sub-word drivers is sufficiently small. Morespecifically the following methods are utilized.

In a first method, three respective of main word lines and common wordlines are set in pairs. A first main word line is connected to the gateof the first PMOS transistor among the sub-word drivers. A first commonword line is connected to the source of the a first PMOS transistor, andwhen the first PMOS transistor is conducting, the first word linevoltage is applied from the first common word line, to the applicablesub-word line by way of the first PMOS transistor drain.

In a second method, the second main word line among the main word linesis connected to the gate of the first NMOS transistor among the sub-worddrivers, and the source of the first NMOS transistor connected to thestandby voltage −VB. When the second NMOS transistor is conducting, thesecond word line voltage is applied to the sub-word line by way of thedrain of the first NMOS transistor.

In a third method, a second common word line from among the common wordlines is connected to the gate of the second NMOS transistor from amongthe sub-word drivers. The source of the second NMOS transistor isconnected to the standby voltage −VB. When the second NMOS transistor isconducting, the second word line voltage is applied to the sub-word lineby way of the drain of the second NMOS transistor.

In a fourth method, the third main word line from among the main wordlines is connected to the gate of the third NMOS transistor from amongthe sub-word drivers. A third word line voltage is applied to the sourceof the third NMOS transistor. A third common word line of the commonword line is connected to the gate of the fourth NMOS transistor fromamong the sub-word drivers. The drain of the third NMOS transistor isconnected to the source of the fourth NMOS transistor, and when thethird NMOS transistor and fourth NMOS transistor are both conducting,the third word line voltage is applied to the sub-word line by way ofthe fourth NMOS transistor, so the voltage across the gate and drain ofthe third NMOS transistor is in this way reduced.

In a fifth method, a first main word line of the main word lines isconnected to the gate of the first PMOS transistor among the sub-worddrivers. A second main word line of the main word lines is connected tothe gate of the first NMOS transistor among the sub-word drivers. Thevoltages for the gate electrode of the first PMOS transistor and thegate electrode of the first NMOS transistor are separated, and thevoltage across the gate and source of the applicable MOS transistors inthis way reduced.

In a sixth method, a first common word line of the common word lines isconnected to the source of the first PMOS transistor among the sub-worddrivers. A third common word line of the common word lines is connectedto the gate of the fourth NMOS transistor among the sub-word drivers.The voltage of the gate electrode third NMOS transistor and the sourceelectrode of the first PMOS transistor are separated, and the voltageacross the gate and source of the first PMOS transistor and the voltageacross the gate and drain of the fourth NMOS transistor are in this wayreduced.

In a seventh method, a second PMOS transistor applied with a fixedvoltage at the gate electrode, is inserted between the applicablesub-word line and the drain of the first PMOS transistor of the sub-worddrivers, and the voltage across the gate and drain of the applicablePMOS transistor in this way reduced.

In an eighth method, a fifth NMOS transistor applied with a fixedvoltage at the gate electrode is inserted between the applicablesub-word line and the drain of the first NMOS transistor of the sub-worddrivers, or the drain of a second NMOS transistor among the sub-worddrivers, and the voltage across the gate and drain of the applicableNMOS transistor in this way reduced.

By a combination of the above eight methods, the sub-word driver cangenerate a three-value word line voltage. Further, the voltage appliedto the gate oxidation film of the MOS transistors comprising thesub-word driver can be kept sufficiently small.

The first embodiment is as described next. A detailed example of thefirst embodiment is explained as shown below.

A semiconductor device has a plurality of word lines, a plurality ofdata lines installed to intersect with the plurality of word lines, aplurality of memory cells installed at the desired cross points of theplurality of data lines and plurality of word lines, a plurality of worddrivers to drive the plurality of word lines, and each of the pluralityof word drivers (SWD) is comprised of a first conduction first MOStransistor (Mp1) supplied at either the drain or the source with a firstvoltage (VW), a second conduction first MOS transistor (Mn1) applied ateither the drain or the source with a second voltage (−VB), a secondconduction second MOS transistor (Mn2) applied at either the drain orthe source with a second voltage (−VB), a second conduction third MOStransistor (Mn3) applied at either the drain or the source with a thirdvoltage (VR), a second conduction fourth MOS transistor (Mn4) connectedat either the drain or source, to either of the other drain or source ofa second conduction third MOS transistor (Mn3), and characterized inthat each of the plurality of word drivers outputs any of a firstvoltage, a second voltage or a third voltage.

A semiconductor device of the second embodiment, according to the firstembodiment is characterized in that each of the plurality of worddrivers outputs a first voltage to the applicable word line when thefirst conduction first MOS transistor is conducting, and outputs a thirdvoltage to the applicable word line when the second conduction third MOStransistor as well as the fourth MOS transistor are conducting, and inother cases a second voltage is output to the applicable word line.

The third embodiment is as follows.

A specific example of the third embodiment is shown in FIG. 1.

A semiconductor device of the first embodiment, characterized in thatthe plurality of word drivers comprise a first conduction second MOStransistor (Mp2) connected with the word line and with the other sourceor drain of the first conduction first MOS transistor (Mp1), and asecond conduction fifth MOS transistor (Mn5) connected between the wordline and the other source or drain of the second conduction first MOStransistor (Mn1) as well as the second conduction second MOS transistor(Mn2), and a fourth voltage (Vss) is applied to the gate of the firstconduction second MOS transistor (Mp1) and a fifth voltage (VDL) isapplied to the gate of the fifth MOS transistor.

Here, a circuit need not always comprise the Mp1 and Mn5 transistorswhich may not be required.

The fourth embodiment is as follows. A specific example is described bymeans of the second embodiment.

In this example, a semiconductor device having a plurality of wordlines, a plurality of data lines intersecting with the plurality of wordlines, a plurality of memory cells installed at desired cross points ofthe plurality of data lines and plurality of words lines, and aplurality of word drivers installed to drive each of the plurality ofword lines, and each of the plurality of word drivers has a firstconduction first MOS transistor (Mp1) supplied at either the source orthe drain with a first voltage (VW) for a desired period, a secondconduction first MOS transistor (Mn1) applied at either the source orthe drain with a second voltage (−VB), a second conduction second MOStransistor (Mn2) applied at either the source or the drain with a secondvoltage (−VB), a second conduction third MOS transistor (Mn3) applied ateither the source or the drain with a third voltage (VR) for a desiredperiod, and the semiconductor device is characterized in that each ofthe plurality of word drivers outputs any one voltage selected fromamong the first voltage, the second voltage or third voltage.

The semiconductor device of the fifth embodiment according to the fourthembodiment, is characterized in that each of the plurality of worddrivers; outputs a first voltage (VW) to the applicable word line when afirst voltage VW is supplied to the drain or source of the firstconduction first MOS transistor (Mp1); outputs a second voltage (−VB) tothe applicable word line when a second conduction first MOS transistor(Mn1) or a second conduction second MOS transistor (Mn2) are conducting;and outputs a third voltage (VR) to the applicable word line when thesecond conduction third MOS transistor is conducting and a third voltage(VR) is applied to the source or the drain of that second conductionthird MOS transistor, and in other cases a second voltage (−VB) isoutput to the applicable word line.

The sixth embodiment is described next. A specific example is given byway of embodiment 3 or embodiment 4.

In this example, a semiconductor device is comprised of a plurality ofword lines, a plurality of data lines intersecting with the plurality ofword lines, a plurality of memory cells installed at desired crosspoints of the plurality of data lines and plurality of words lines, anda plurality of word drivers installed to drive each of the plurality ofword lines; and each of the plurality of word drivers is comprised of afirst conduction first MOS transistor supplied at either the source orthe drain with a first voltage (VW) in a first period and a thirdvoltage (VR) at a second period, a second conduction first MOStransistor applied at either the source or drain with a second voltage(−VB), a second conduction second MOS transistor applied at the sourceor drain for at least a specified desired period with a second voltage(−VB) and the semiconductor device is characterized in that each of theplurality of word drivers outputs any one voltage selected from amongthe first voltage, the second voltage or third voltage.

A fixed voltage input to the source or drain of the transistor Mn2 willalso serve adequately here as the second voltage (−VB).

The semiconductor device of the seventh embodiment according to thesixth embodiment is characterized in that each of the plurality of worddrivers outputs a first voltage (VW) to the applicable word line whenthe first conduction first MOS transistor is conducting; outputs a thirdvoltage (VR) to the applicable word line when the first conduction firstMOS transistor is conducting in a second period; and in other casesoutputs a second voltage (−VB) to the applicable word line.

The semiconductor device of the eighth embodiment according to the sixthembodiment, has a plurality of word drivers comprising a firstconduction second MOS transistor (Mp2) between the word line and theother source or drain of the first conduction first MOS transistor, asecond conduction fifth MOS transistor (Mn5) between the word line andthe other source or drain of the second conduction first MOS transistorand the second conduction second MOS transistor, and furthercharacterized in that a fourth voltage (VSS) is applied to the gate ofthe first conduction second MOS transistor, and a fifth voltage (VDL) isapplied to the gate of the second conduction fifth MOS transistor.

The semiconductor device of the ninth embodiment according to the firstthrough eighth embodiments is characterized in that the materialcomprising the region contacting the first conduction first MOS gatetransistor oxidation film is different from the material comprising theregion contacting the first conduction MOS gate transistor oxidationfilm contained in circuits to drive the gate electrode of the firstconduction first MOS transistor.

The semiconductor device of the tenth embodiment according to the firstthrough eighth embodiments is characterized in that first voltage (VW)is larger than the third voltage (VR), and that the third voltage islarger than the second voltage (−VB).

The semiconductor device of the eleventh embodiment according to thethird through eighth embodiments is characterized in that first voltage(VW) is larger than the third voltage (VR), and that the third voltageis larger than the second voltage (−VB), and that the fourth voltage(VSS) is a voltage level between the second voltage and the thirdvoltage, and that the fifth voltage (VDL) is a voltage level between thefirst voltage and the third voltage.

The semiconductor device of the twelfth embodiment according to thefirst through eighth embodiments is characterized in that each of theplurality of memory cells perform the write operation when theapplicable word line is a first voltage; holds the data when theapplicable word line is a second voltage; and perform read operationwhen the applicable word line is at a third voltage.

The semiconductor device of the thirteenth embodiment according to thefirst through eighth embodiments is a dynamic three-transistor cell,characterized in that the plurality of memory cells are a first MOStransistor connected at the gate to the word line and having either asource or a drain connected to the data line, a second MOS transistorconnected at the gate to either of the sources or drains of the firstMOS transistor, and a third MOS transistor connected at the gate to theword line and connected at either of the drains or sources to either ofthe drains or sources of the second MOS transistor.

The semiconductor device of the fourteenth embodiment according to thefirst through eighth embodiments is a dynamic capacitive coupling2-transistor cell, characterized in that the plurality of memory cellsare a first MOS transistor connected at the gate to the word line andhaving either a source or a drain connected to the data line, a couplingcapacitor connected to one terminal of the word line, and a second MOStransistor with a gate connected to the other terminal of the couplingcapacitor and to either of the source or the drain of the first MOStransistor.

As is the normal practice, the first conduction type is a P type and thesecond conduction type is an N type.

The embodiments of the invention are hereafter described in detail.

First, the embodiment of the invention when utilizing a capacitivecoupling 2-transistor cell in the memory cell is described in detail.

The following explanation assumed the voltage settings shown in FIG. 6.Typical voltage settings for a DRAM utilizing a capacitor coupledtransistor cell are shown in FIG. 6. The upper and low positions of thevoltage high and low points are shown in the figure. In other words, thesupply voltage is set as VDL, the high level of the bit lines, senselines and peripheral circuits are set as VDL, the low level of the bitlines, sense lines and peripheral circuits are set as ground potentialVSS, the first high level of the main word line and common word linesset as VW (hereafter write voltage), the first low level of the mainword line and common word lines set as ground potential VSS, the secondhigh level of the main word line and common word line set as supplyvoltage VDL, the second low level of the main word line and common wordline set as −VB (hereafter standby voltage), the first high level of thesub-word line set as the write voltage VW, the sub-word line low levelset as standby voltage −VB, and the second high level of the sub-wordline (hereafter read voltage) is set as VR.

The maximum electric field strength in the oxidation film of the MOStransistor must generally be set as Eox max=4.5 [MV/cm] from theviewpoint of reliability of the gate insulation film. The allowable gateoxidation film thickness allowed in the PMOS transistor and NMOStransistor of the sub-word driver at this time is expressed as toxp andtoxn. The absolute value of the PMOS transistor and NMOS transistorthreshold voltages are assumed here to respectively be |Vthp|=−0.3 [V]and |Vthn|=0.3 [V].

Also in these specifications, when there is no particular explanation tothe contrary, the gate electrode material of the PMOS transistor withinthe peripheral circuit is normally P plus silicon doped with asufficient concentration of acceptors (hereafter, p+Si), and for theNMOS transistor is N plus silicon sufficiently doped with donors(hereafter n+Si). This material is intended to lower the MOS transistorthreshold voltage without using a larger ion implantation quantity toadjust the threshold voltage. The gate electrode material here, ismaterial for the portion contacting the gate oxidation film within thegate electrode. For example, the gate material was described above asp+Si, there is no need for the entire gate to be p+Si and for instance adual layer structure of p+Si and a high fusing point material such astungsten may be utilized.

In such a case, when the supply voltage of the peripheral circuits isset as VDL=1.5 [V], then the allowable gate oxidation film thickness toxin the peripheral circuits is set as,tox=VDL÷Eox max=1.5 [V]÷4.5 [MV/cm]≈3.3 [nm]

However, in actual operation, the thickness must be set to a degree thatprevents the occurrence of tunnel current flowing in the gate oxidationfilm, and is estimated as approximately 5 [nm].

<First Embodiment>

The first embodiment is described while referring to FIG. 1, and FIG. 7through FIG. 12. The drawing in FIG. 1 shows the configuration of thesub-word driver for driving the sub-word lines to the three voltagevalues. FIG. 7 shows a typical configuration of the DRAM hierarchicalword line structure of this invention. FIG. 8 is a drawing illustratingthe operation of the sub-word driver of FIG. 1. FIG. 9 is a drawingshowing the configuration of the main word driver circuit for thesemiconductor storage device. FIG. 10 is a drawing showing the commonword driver structure. FIG. 11 is a drawing showing a typical memorycell array utilizing the capacitive coupling 2 transistors shown in FIG.5. FIG. 12 is a drawing showing the operation timing of the memory cellcomprised by utilizing two transistors and one capacitor.

The hierarchical word structure is described next while referring toFIG. 7. The sub-word drivers SWD (SWD111, SWD112, . . . ) respectivelycontrolling the separate sub-word lines SWL (SWL111, SWL112 . . . ) arerespectively installed at cross points of the main word lines MWLbp(MWL1 bp, MWL2 bp, . . . ), MWLbn (MWL1 bn, MWL2 bn, . . . ), MWLRtn(MWLR1 tn, MWLR2 tn, . . . ) and common word lines FXtp (FX11 tp, FX12tp, . . . ) FXtn (FX11 tn, FX12 tn, . . . ), FXbn (FX11 bn, FX12 bn, . .. ). These sub-word drivers SWD are comprised of a plurality of units ofsub-word driver arrays SWDA (SWDA11, SWDA12, . . . ).

The sub-word lines SWL are connected to the memory cell array MCA(MCA11, MCA12, . . . ) The write control circuit arrays RWCA (RWCA1,RWCA2, . . . ) comprised of a plurality of units of write controlcircuits RWC (RWC11, RWC12, . . . ) are located adjacent to these memorycell arrays. The main word lines MWLbp, MWLbn, MWLRtn are driven by themain word drivers MWD (MWD1, MWD2, . . . ), and cross above the sub-worddriver arrays SWDA and the memory cell arrays MCA.

The main word lines here are comprised of complementary true(non-inverted) and bar (inverted) signals and identified from each otherby the appended letters t and b of the reference signals. The (true)non-inverted signals are for the PMOS transistors and the (bar) invertedsignals are for the NMOS transistor signals, and are identified by theappended letters p and n of the respective reference signals. The setsof common word lines FXtp, FXtn as well as FXbn are driven by the commonword drivers FXD (FXD11, FXD12, . . . ) and their common word driversFXD are comprised of a plurality of units of common word driver arraysFXDA (FXDA1, FXDA2, . . . ). The main word driver array MWDA and commonword driver array FXDA are installed at the periphery of the sub-worddriver array SWDA and memory cell array MCA as well as write controlcircuit array RWCA.

The relation of sub-word line and memory cell, is where the memory cellis connecting with the sub-word line at the position shown by the whitecircles at the intersection of the sub-word lines SWL and data lines DLin the memory cell arrays MCA (MC11, MC12, . . . ).

This memory cell is the three-transistor cell shown in FIG. 4 asdescribed above. In the case of the capacitive coupling 2-transistorcell shown in FIG. 3, a bit line BL and a sense line SL are installedinstead of the data line DL. The write control circuits RWC (RWC11,RWC12, . . . ) are connected at the end of the data line DL (DL11, DL12,. . . ).

Though not shown in FIG. 7, the circuit of FIG. 7 is provided with anaddress input signal terminal and an address decoder for controllingselection of the memory cell that performs read operation, and thiscircuit also issues a decode signal decoded from the address signal thatwas input to the address decoder. This decode signal functions toactivate the main word driver MWD and common word driver FXD forspecifying the sub-word line SWL contained in the selected memory cell.

<Sub-Word Driver Structure>

The structure of the sub-word driver SWD of this invention for drivingthe sub-word lines to the three voltage values is shown in FIG. 1. Inthis figure, signals for conduction by majority carriers in P type MOStransistors are identified by means of the attached arrows and thesignals for the N type MOS transistors are identified by having noarrows.

The PMOS transistor Mp1 and the NMOS transistor Mn1 are handled withseparate main word line signals. The gate of the PMOS transistor Mp1 isconnected to main word line MWLbp, while the gate of the NMOS transistorMn1 is connected to the main word line MWLbn. The gate of the NMOStransistor Mn3 is also connected to the main word line MWLRtn. The PMOStransistor Mp1 and the NMOS transistor Mn4 are also handled withseparate common word lines. The source of the PMOS transistor isconnected to the common word line FXtp, and the gate of the NMOStransistor Mn4 is connected to the common word line FXtn. The gate ofthe NMOS transistor Mn2 is connected to the common word line FXbn. Thesources of the NMOS transistors Mn1 and Mn2 are connected to the standbyvoltage −VB, and a read voltage VR is input to the source of the NMOStransistor Mn3. The PMOS transistor Mp2 and the NMOS transistor Mn5 areoxide-stress relaxation MOS transistors, and a fixed voltage is input tothe gate electrodes. FIG. 1 shows respective application of groundpotential VSS and supply voltage VDL. The transistor Mn4 also fulfillsthe function of an oxide-stress relaxation MOS transistor. The sub-wordline SWL is connected to the drains of the transistors Mp2, Mn4 and Mn5.

<Sub-Word Driver Operation>

The operation of the sub-word driver SWD of FIG. 1 is explained whileaccording to FIG. 8.

FIG. 8 shows the operation when the sub-word line SWL111 is selected.The read operation and write operation are carried out in successionfrom standby state. First of all, when the read control signal φr atground potential VSS is driven to read status at supply voltage levelVDL, the main word driver MWD1 drives the main word line MWL1 bn fromsupply voltage level VDL to standby voltage −VB and the main word lineMWLR1 from standby voltage −VB too supply voltage VDL. Also, the commonword driver FXD11 drives the common word line FX11 tp and FX11 tn fromrespectively ground potential VSS and standby voltage −VB, torespectively the write voltage VW and the supply voltage level VDL. Themain word line MWL1 bp is therefore driven to the write voltage VW, themain word line MWL1 bn is driven to the standby voltage −VB, and themain word line MWLR1 tn is driven to the supply voltage VDL. Also, thecommon word line FX11 tp is driven to the write voltage VW, the commonword line FX11 tn is driven to supply voltage level VDL and the commonword line FX11 bn is driven to the standby voltage −VB so that thetransistors Mn3, Mn4 start conduction and the sub-word driver SWD111 isselected and the sub-word line SWL111 is driven from the standby voltage−VB to read voltage VR.

Next, when the read control signal φr at supply voltage level VDL isdriven to ground potential VSS, the main word driver MWD1 drives themain word line MWL1 bp from write voltage VW to ground potential VSS,and the main word line MWLR1 tn at the supply voltage VDL is driven tostandby voltage −VB. The main word line MWL1 bp is therefore driven toground potential VSS, the main word line MWL1 bn is driven to thestandby voltage −VB, the main word line MWLR1 tn is driven to standbyvoltage −VB. Further, the common word line FX11 tp is driven to thewrite voltage VW, the common word line FX11 tn is driven to supplyvoltage VDL, and the common word line FX11 bn is driven to the standbyvoltage −VB so that the transistors Mp1, Mp2 start to conduct and thesub-word driver SWD111 is selected. This sub-word line SWL111 at theread voltage VR, is driven to the write voltage VW.

In this way, in the operation that selects the sub-word driver SWD111,the non-selected sub-word drivers are in the following three states. Inother words, firstly, the main word line and the common word line areboth in non-select status, secondly the common word line selected by themain word line is in non-select status, and thirdly with the main wordline in non-select status the common word line is in select status.Hereafter, these states are explained in order.

Firstly, the state when the main word line and the common word line areboth in non-select status is explained. During standby, all sub-worddrivers SWD are in this non-select status. When the sub-word driverSWD111 is selected, the sub-word driver SWD221 for instance, maintainsthis same status during standby. Generally, at this point, in thesub-word driver SWD during standby, the main word line MWLbp is drivento write voltage VW, the main word line MWLbn is driven to supplyvoltage VDL, the main word line MWLRtn is driven to standby voltage −VB,also the common word line FXtp is driven to ground potential VSS, thecommon word line FXtn is driven to standby voltage −VB, and the commonword line FXbn is driven to supply voltage VDL so that the transistorsMn1, Mn2 for the sub-word drivers SWD start to conduct (turn on), andthe transistors Mp1, Mn3, Mn4 turn off, and the sub-word line SWL isheld at standby voltage −VB.

Secondly, the states when the main word line is selected and the commonword lines are not selected is explained. When the sub-word driverSWD111 is selected, the sub-word driver SWD121 sets to this status. Theoperation of the sub-word driver SWD121 is shown in the middle sectionof FIG. 8.

When the read control signal φr at ground potential VSS is driven tosupply voltage level VDL and sets to read status, the main word driverMWD1 drives the main word line MWL1 bn at supply voltage level VDL toground potential VSS, and the main word line MWLR1 tn at standby voltage−VB is driven to supply voltage VDL. The common word driver FXD21 isheld at non-select status, and the common word lines FX21 tp, FX21 tnand FX21 bn are held at ground potential VSS, standby voltage −VB andsupply voltage VDL. Therefore, the main word line MWL1 bp is driven towrite voltage VW, the main word line MWL1 bn is driven to standbyvoltage −VB, and the main word line MWLR1 tn is driven to supply voltageVDL, also the common word line FX21 tp is driven to ground potentialVSS, the common word line FX21 tn is driven to standby voltage −VB, thecommon word line FX21 bn is driven to supply voltage VDL so that thetransistors Mn2, Mn3 for the sub-word driver SWD121 start to conduct(turn on), and the transistors Mp1, Mn1, Mn4 turnoff, the sub-word lineSWL121 is held at standby voltage −VB.

Next, when the read control signal φr drops from supply voltage VDL toground potential VSS reaching write status, the main word driver MWD1drives the main word line MWL1 bp from write voltage VW to groundpotential VSS, the main word line MWL1 tn at supply voltage level VDL isdriven to standby voltage −VB. The main word line MWL1 bp is thereforedriven to ground potential VSS, the main word line MWL1 bn is driven tostandby voltage −VB and the main word line MWLR1 tn is driven to groundpotential VSS, also the common word line FX21 tp is driven to groundpotential VSS, the common word line FX21 tn is driven standby voltage−VB and the common word line FX21 bn is driven to supply voltage levelVDL so that the transistor Mn2 for the sub-word driver SWD121 start toconduct (turn on), the transistors Mp1, Mn1, Mn3, Mn4 turnoff, and thesub-word line SWL121 is next held at standby voltage −VB.

Thirdly, the state when the main word line is at non-select status andthe common word line is in select status are explained. When thesub-word driver SWD111 is selected, the sub-word driver SWD211 sets tothat status. The operation of the sub-word driver SWD121 here is shownin the lower section of FIG. 8.

When the read control signal φr at ground potential VSS is driven toread status at supply voltage level VDL, the main word driver MWD1 isheld at non-select status, and the main word lines MWL2 bp, MWL2 bn andMWLR2 n are held at write voltage VW, supply voltage VDL and standbyvoltage −VB. Also the common word driver FXD11 drives the common wordlines FX11 tp, FX11 tn from ground potential VSS, standby potential −VB,to respectively write voltage VW and supply voltage level VDL. The mainword line MWL2 bp is therefore driven to write voltage VW, the main wordline MWL2 bn to supply voltage level VDL, and the main word line MWLR2tn to standby voltage −VB, also the common word line FX11 tp is drivento write voltage VW, the common word line FX11 tn is driven to supplyvoltage VDL and the common word line FX11 bn is driven to standbyvoltage −VB so that the transistors Mn1, Mn4 for the sub-word-driverSWD211 start to conduct (turn on), the transistors Mp1, Mn2, Mn3, turnoff, and the sub-word line SWL211 is next held at standby voltage −VB.Also, even if the read control signal φr at supply voltage VDL is drivento ground potential VSS, the status of main word lines MWL2 bp, MWL2 bnand MWLR2 tn, and the status of the common word lines FX11 tp, FX11 tnand FX11 bn are maintained, and the sub-word line SWL211 is maintainedat standby voltage −VB by the sub-word driver SWD211.

A case showing the application of voltage to the gate oxidation film ofeach MOS transistor in sub-word driver SWD111 configured as shown inFIG. 1 is shown based on the above described operation. This example, isdescribed using an NMOS transistor when the supply voltage is set asVDL=1.5 [V], the standby voltage is set as −VB=[−2V], the read voltageis set as VR=0.5 [V], and the write potential is set as VW=3[V].

The second high level of supply voltage VDL is input to the gate of theMOS transistor Mn5 in the sub-word driver SWD11 that was selected sothat during the read operation, the voltage applied to the gate anddrain of the MOS transistor Mn5 becomes:VW−VDL=1.5 [V]

Further, current does not flow constantly to the transistor Mn5 sincethe NMOS transistors Mn1, Mn2 are in the cutoff state, and the sourcevoltage of the transistor Mn5 becomes (VDL−Vthn). Therefore the voltageapplied to the gate oxidation film between the gate and source of thetransistor Mn5 becomes,VDL−(VDL−Vthn)=0.3 [V]and during the write operation, the voltage applied to the gateoxidation film between the gate and drain of the NMOS transistors Mn1,Mn2 becomes,(VDL−Vthn)−(−VB)=3.2 [V]

Therefore, by inserting a transistor Mn5 input at the gate with a supplyvoltage VDL, the drain voltage potential of the transistor Mn1 is pulleddown from the write voltage VW to the (VDL−Vthn) so that the voltageapplied to the gate oxidation film between the gate and drain of thetransistors Mn1, Mn2 is reduced by an amount equal to,VW−(VDL−Vthn)=1.8 [V]

Also, during the write operation, by inputting VDL to the gate of theMOS transistor Mn4 from the common word line FXtn, the same reasoningapplies to the voltage applied between the gate and drain as well asbetween the gate and source of the transistors Mn3 and Mn4, so that thebreakdown voltage can be reduced.

On the other hand, in regards to the standby status and non-selectstatus of the sub-word driver, by separating the main word lines MWLbpand MWLbn from the common word lines FXtp and FXnp, the voltage input tothe transistors Mn1, Mn2 is reduced by an amount equal to,VW−VDL=1.5 [V]and the breakdown voltage can be reduced. In other words, the voltageinput to the transistors Mn1, Mn2 is reduced just by the above amount,and the voltage applied between the gate and drain, and between the gateand source is a maximum for the transistors Mn1, Mn2 and Mn5 atVDL−(−VB)=3.5 [V]

Therefore, in a sub-word driver configured as shown in FIG. 1, the gateoxidation film toxn is set thicker than,(VDL+VB)÷Eox max=3.5 [V]÷4.5 [MV/cm]≈7.8 [nm]

so as not to exceed the maximum electric field of 4.5 [MV/cm]. The gateoxidation film toxn the NMOS transistor in the sub-word driver is setwithin this range, and the problem of breakdown voltage on the gateoxidation film between the gate and drain of transistors Mn1, Mn2 can inthis way be resolved. From these results and from the previously relatedtox figures, if the sub-word driver and peripheral circuits aredifferentiated per the gate oxidation film thickness, then high speedcircuit operation can be achieved.

On the other hand, making the film thickness of the peripheral circuitsmatch the sub-word driver value allows simplifying the manufacturingprocess and the number of masks required can be reduced. In some cases,the second high level (here, the supply voltage VDL) of the main wordline MWLbn and the voltage level input to the gate of the transistor Mn5can be set as an appropriate value not exceeding the maximum electricfield of 4.5 [MV/cm], and the voltage level input to the gate of thetransistor Mn5 may be set as a pulse signal having an appropriateamplitude. However, the high level of the data line DL is preferably setto the same level as the supply voltage VDL so as not to increase thenumber of supply voltage lines, and lighten the load on the power supplywithin the chip, in order to keep the drive capability of the transistorMn5 about the same level as the transistors Mn1 and Mn2.

The case of the PMOS transistor in next explained. By separating themain word lines MWLbp and MWLbn, in the selected sub-word driver SWD111,the voltage input to the gates of the transistors Mp1, Mp2 can bereduced by an amount equal to,VSS−(−VB)=2 [V]and the breakdown voltage can be reduced. In other words, the voltageinput to the transistors Mp1, Mp2 is reduced by this amount, and thevoltage differential between the gate and source and between the gateand drain of PMOS transistors Mp1, Mp2 becomes a maximum during thewrite operation, and the write voltage VW equals 3 [V]. In standbystatus and non-select status on the other hand, a fixed input at groundpotential VSS is input to the gate of the transistor Mp2 so that thevoltage applied to the gate oxidation film between the gate and drain ofthe transistor Mp2 becomes,VSS−(−VB)=2 [V]

Also, current does not flow to the transistor Mp2 since the transistorMp1 is turned off, and the source voltage of the transistor Mp2 becomes,VSS+|Vthp|=0.3 [V]so that the voltage differential between the gate and source oftransistor Mp2 becomes, (VSS+|Vthp|)−VSS=0.3 [V]

The voltage applied to the gate oxidation film between the source anddrain of PMOS transistor Mp1 therefore becomes,VW−|Vthp|=2.7 [V]

Therefore, by inserting transistor Mp2 input at the gate with groundpotential VSS, the drain voltage potential of transistor Mp1 is pulleddown from write voltage −VB to threshold voltage |Vthp| so that thebreakdown voltage can be reduced. In other words, the voltage applied tothe gate oxidation film between the gate and drain is reduced by anamount equal to,(VW+VB)−(VW−|Vthp|)=2.3 [V]

Therefore, in a sub-word driver configured as shown in FIG. 1, the gateoxidation film toxp of the PMOS transistor is set thicker than,VW÷Eox max=3 [V]÷4.5 [MV/cm]≈6.7 [nm]

so as not to exceed the maximum electric field of 4.5 [MV/cm]. Theproblem of breakdown voltage on the gate oxidation film between the gateand drain of transistors Mp1, Mp2 can in this way be resolved. Fromthese results and from the tox figures related previously, if thesub-word driver and peripheral circuits are differentiated per the gateoxidation film thickness, then high speed circuit operation can beachieved.

On the other hand, making the film thickness of the peripheral circuitsmatch the sub-word driver value allows simplifying the manufacturingprocess and the number of masks required can be reduced. In some cases,the first low level (here, the ground potential VSS) of the main wordline MWLbp and the voltage level input to the gate of the transistor Mn5can be set as an appropriate value not exceeding the maximum electricfield of 4.5 [MV/cm], and the voltage level input to the gate of thetransistor Mn5 may be set as a pulse signal having an appropriateamplitude. However, the low level of the data line DL is preferably setto the same level as ground potential VSS so as not to increase thenumber of supply voltage lines, and lighten the load on the power supplywithin the chip, in order to keep the drive capability of the transistorMp2 about the same level as the transistors Mp2.

Also, when the gate electrode material of the transistors Mp1, Mp2 iscombined by the method using n+si, the voltage applied to the gateoxidation film between the gate and drain of the transistor Mp2 can bereduced approximately one volt, equivalent to the work functiondifferential ΔW with the drain electrode of p+Si, and the gate oxidationfilm can thus be made even thinner.

The features related above for the sub-word driver shown in FIG. 1 arenow summarized.

(1) In this circuit structure, a select or non-select signal can beissued for a voltage level corresponding to the memory cell read orwrite operation by utilizing a decode signal in the hierarchical wordline structure of the related art. In other words, by inserting an NMOStransistor Mn3, Mn4, the selected sub-word line can be driven to theread voltage VR during the read operation and to the write voltage VWduring the write operation. Also, when maintaining standby status ornon-select status, the applicable sub-word line can be held at thestandby voltage −VB.

(2) Also in this circuit structure, the electric field applied to thegate oxidation film of the MOS transistor can be reduced, regardless ofthe select or non-select state. In other words, by inserting anoxide-stress relaxation PMOS transistor Mp2 and NMOS transistor Mn5, theproblem of breakdown voltage in the gate oxidation film between the gateand drain in PMOS transistor Mp1 and NMOS transistors Mn1, Mn2 can beeliminated.

(3) By isolating the main word line MWL signal in MWLbp and MWLbn ofdifferent voltage amplitudes, and by isolating the main word line FXsignal in FXtp and FXtn of different voltage amplitudes, the problem ofbreakdown voltage in the gate oxidation film between the gate and sourceof the MP1 transistor in the selected sub-word driver can be resolved,and the problem of breakdown voltage in the gate oxidation film betweenthe gate and source of the Mn1, Mn2 transistors in the non-selectedsub-word driver can also be resolved. Also, the problem of breakdownvoltage in the gate oxidation film between the gate and drain of the Mn3transistors in the non-selected sub-word driver can also be resolved.

(4) Further, the problem of breakdown voltage in the gate oxidation filmbetween the gate and drain of the MP1 transistor in the selectedsub-word driver can be resolved by applying the method of raising thethreshold voltage with n+Si material having a work functionapproximately 1 volt lower than the p+Si gate electrode material oftransistor Mp1. Accordingly, along with resolving the breakdown voltageproblem, the sub-word driver driving the sub-word line to three voltagevalues can be comprised of seven MOS transistors.

The main word driver MWD and the common word driver FXD for respectivelydriving the main word lines MWLbp, MWLbn and MWLRtn and the common wordlines FXtp, FXtn and FXbn connected to the sub-word driver shown in FIG.1 are explained next.

<Main Word Driver>

A typical circuit structure of a main word driver is shown in FIG. 9.The voltage amplitude of the main word line from −VB to VW must belarger than the voltage amplitude of the peripheral circuits which isfrom VSS to VDL, so by utilizing the sub-word driver shown in FIG. 1,the voltage amplitude of the peripheral circuits is level shifted perthe main word driver. Also, the problem of breakdown voltage on the gateoxide film between the gate and drain as well as the gate and source ofthe sub-word driver transistors Mp1, Mn1 and Mn2 is resolved, and thethree types of main word lines MWLbp, MWLbn and MWLRtn are utilized togenerate the voltage level select signal for the memory cell read andwrite operation. The main word driver MWD is therefore comprised of thelevel shift circuits LSCH, LSCL1 and LSCL2 and the read/write controlcircuit RWCC1 for independently driving the main word lines MWLbp, MWLbnand MWLRtn.

The read/write control circuit RWCC1 is first explained. The decodesignal axj is input to a first input terminal of a NOR circuit NR1 byway of the inverter circuit NV1, and the read/write control signal φrinput to the second input terminal of NR1. The decode signal axj is alsoinput to the first input terminal of the NAND circuit ND1, and theread/write control signal φr input to the second input terminal of ND1.The output of NR1 is set as the decode signal axjr11 and the output ofND1 is the decode signal axjr12.

Next, the first level shift circuit LSCH is explained. This circuittakes an input signal having a voltage amplitude from ground potentialVSS to supply voltage VDL and outputs it as an output signal having avoltage amplitude with a higher level (here, write voltage VW) thanground potential VSS to supply voltage VDL. The decode signal axjr11 isinput to the gate of the NMOS transistor Mn1 and the source of the NMOStransistor Mn2, and the source of the transistor Mn1 is grounded. Thedrain of transistor Mn1 and PMOS transistor Mp1 as well as the gate ofMp2 are grounded to the first main word line MWLbp. The write voltage VWis input to the source of the transistors Mp1, Mp2, and the drain oftransistors Mn2, Mp2 are connected to the gate of the transistor Mp1 anda feedback circuit formed. Here, by inputting supply voltage VDL to thegate of transistor Mn2, the when the output of the main word line MWLbpreaches ground potential VSS, the DC current flow by way of thetransistor Mp2 is cutoff.

Further, the level shift circuit LSCL1 is shown from among the secondlevel shift circuits LSCL1 and LSCL2. The level shift circuits LSCL1 andLSCL2 have the same circuit structure and level shift an input signalhaving a voltage amplitude from ground potential VSS to supply voltageVDL, and output it as a signal having a voltage amplitude of a level(here, standby voltage −VB) lower than ground potential VSS to supplyvoltage VDL.

A decode signal axj is input to the gate of PMOS transistor Mp1 and thesource of PMOS transistor Mp2, and supply voltage VDL input to thesource of the transistor Mp1. The drain of transistor Mp1 and NMOStransistor Mn1 as well as the gate of Mn2 are connected to the secondmain word line MWLbn. Also, the source of transistors Mn1, Mn2 areconnected to standby voltage −VB, and the drains of transistor Mp2, Mn2are connected to the gate of the transistor Mn1 to form a feedbackcircuit. Here, by inputting a ground potential VSS (level) to the gateof transistor Mp2, the DC current flow by way of transistor Mn2 is shutoff when the output of the main word line MWLbn reaches the supplyvoltage VDL level.

<Main Word Driver Operation>

The operation of the main word driver MWD utilizing the above structureis explained. The main word driver MWD is selected by the decode signalaxj reaching the supply voltage level VDD. Then, the three types of mainword lines MWLbp, MWLbn, and MWLtn are driven to a voltage levelaccording to the memory cell read/write operation.

In other words, when the read/write control signal φr at groundpotential VSS, is driven to supply voltage level VDL during readoperation, the decode signal axjr11 at ground potential VSS is input tothe level shift circuit LSCH, the transistor Mp1 conducts and the mainword line MWLbp is held at write voltage VW. A decode signal axj atsupply voltage VDL is input to the level shift circuit LSCL1, and thetransistor Mn1 conducts and the main word line at supply voltage VDL isdriven to standby voltage −VB. A decode signal axjr12 at groundpotential VSS is input to the level shift circuit LSCL2, the transistorMp1 conducts, and the main word line MWLRtn at standby voltage −VB, isdriven to supply voltage level VDL.

On the other hand, when the read/write control signal φr at supplyvoltage level VDL is driven to ground potential VSS during writeoperation, the decode signal axjr11 at supply voltage level VDL is inputto the level shift circuit LSCH, the transistor Mn1 conducts and themain word line MWLbp at write voltage VW is driven to ground potentialVSS. The decode signal axj is still at supply voltage level VDL so thatfor level shift circuit LSCL1, the transistor Mn1 conducts and holds themain word line MWLbn at standby voltage −VB. Further, the decode signalaxjr12 at supply voltage level VDL is input to the level shift circuitLSCL2, the transistor Mn1 conducts, and the main word line MWLRtn atsupply voltage VDL is driven to standby voltage −VB.

The voltages applied to the gate oxidation film of each transistor inthe main word driver used for performing such operations are describednext. The voltage applied to the gate oxidation film between the gateand source and between the gate and drain of the transistor Mp1 for thelevel shift circuit LSCH, becomes VW at maximum in standby operation andin select main word driver read operation. Also, the voltage applied tothe gate oxidation film between the gate and source of transistor Mp2,is a maximum in the write operation of the selected main word driver.Further, the voltage applied to the gate oxidation film between the gateand drain of transistor Mp2 is a maximum in standby status and readoperation of the selected word driver, and is VW in either case.Therefore, the breakdown voltage problem can be avoided if a gateoxidation film thickness and a gate electrode material the same as thePMOS transistor in the sub-word driver shown in FIG. 1 is utilized. Onthe other hand, a voltage applied to the gate oxidation film between thegate and source, and between the gate and drain of transistor Mn1 forthe level shift circuits LSCL1, LSCL2, is a maximum in the readoperation of the selected sub-word driver, and becomes (VDL+VB). Thevoltage applied to the gate oxidation film between the gate and sourceof transistor Mn2 is a maximum in the read operation of the selectedmain word driver, the voltage applied to the gate oxidation film betweenthe gate and drain of transistor Mn2 is a maximum in the write operationof the selected word driver and is also (VDL+VB) Therefore, thebreakdown voltage problem can be avoided if the same gate oxidation filmthickness as the NMOS transistors in the sub-word driver shown in FIG.1.

<Common Sub-Word Driver>

The common sub-word driver FXD is shown in FIG. 10. The voltageamplitude of the common key word line is from −VB to WV and larger thanthe voltage amplitude from VSS to VDL of the peripheral circuits, sothat the voltage amplitude of the peripheral circuit is shifted to theamplitude level of the common word driver by using the sub-word drivershown in FIG. 1. Also the problem of breakdown voltage in gate oxidationfilm between the gate and source and between the gate and drain oftransistor Mp1, Mn2 and Mn4 of the sub-word driver, and the three typesof common word lines FXtp, FXtn and FXbn are utilized to generate aselect signal for a voltage level corresponding to the read operation ofthe memory cell.

The common word line FXbn uses the inverted signal of the common wordline FXtn so that the common word driver FXD to independently drive thecommon word lines FXtp, FXtn is comprised of the level shift circuitsLSCH, LSCL and the inverter circuits NVL, NV1. The level shift circuitsLSCH, LSCL have the same structure as that related for the main worddrivers, so the decode signal axj is input to the level shift circuitsLSCH, and the decode signal ajb generated by way of the inverter circuitNV1 from the decode signal aj, is input to the level shift circuit LSCL.The output of the level shift circuit LSCH is for the common word lineFXtp, and the output of the level shift circuit LSCL is for the commonword line FXbn. The inverter circuit NVL is comprised of PMOS transistorMp1 and NMOS transistor Mn1 but differs from the inverter of theperipheral circuit in that a standby voltage −VB is input to the sourceof the NMOS transistor Mn. The common word line FXbn is connected to thegates of the transistors Mp1 and Mn1, and the drains are connected tothe common word line FXtn.

<Common Word Driver Operation>

The operation of the common word driver FXD utilized in the abovestructure is described next. The common word driver FXD is selected bythe decode signal aj setting to ground potential VSS, the transistor Mp1of the level shift circuit LSCH conducts and the common word line FXtpat ground potential VSS is driven to write voltage VW. Also, the decodesignal ajb at supply voltage VDL is input to the level shift circuitLSCL, the transistor Mn1 conducts, and the common word line FXbn thatreached supply voltage level VDD is driven to standby voltage −VB. Thetransistor Mp1 for the inverter NVL starts conducts by the common wordline FXbn at standby voltage −VB, and the common word line FXtn atstandby voltage −VB is driven to supply voltage VDL.

In the common word driver performing this kind of operation, the voltageamplitude of the input/output signal is the same as the above describedmain word driver, so that the voltage applied to the gate oxidation filmof each transistor is also equivalent to the main word driver.Accordingly, if a common word driver utilizes transistors having thesame gate oxidation film thickness as the PMOS transistor and NMOStransistor in the above mentioned sub-word driver, then the breakdownvoltage problem can be resolved.

<Memory Cell Arrays>

A memory cell array MCA1 utilizing the capacitive coupling 2-transistorcell of FIG. 5 is shown in FIG. 11. The voltage settings are the typicalpreferred voltage settings in a capacitive coupling 2-transistor cellDRAM shown in FIG. 6. For the purposes of simplicity, four memory cellsMC are used for the two bits lines BL1, BL2 and the two sense lines SL1,SL2, and the two sub-word lines SWL111, SWL121, however, a plurality ofbit lines BL, sense lines SL and sub-word lines SWL are respectivelyformed, and a plurality of memory cells MC are formed at desired crosspoints with these various lines.

An example of memory cells MC formed at cross points with these bitlines BL, sense lines SL and sub-word lines SWL is shown in FIG. 11. Aspecified circuit configuration such as having witches to control theoperation timing of read and write circuits formed with bit lines, andsense lines, as well as charging circuits, and switches for input/outputis omitted. The same circuits of the related art are sufficient.

The operation of the memory cell is shown in FIG. 12. First of all, whena write voltage VW with a pulse voltage higher than the thresholdvoltage VTW of the transistor QW is applied to the selected sub-wordline SWL, the transistor QW conducts (turns on), and the voltagepotential of the bit line according to the write data, is applied to thememory cell node N, and writing operation begins. This voltage potentialis provided by way of a sequence selected write circuit applied with anexternal voltage, and for instance, is voltage supply level VDL whenstoring the information “1”, and is ground potential VSS when storingthe information “0”. Next, the sub-word line SWL sets to standby voltage−VB. At this time, the voltage VN(H) of the memory cell node appliedwith supply voltage VDL, becomes lower than the threshold voltage VTR oftransistor QR due to the coupling capacitor Cc so that the transistorQR, QW are cut-off and the information is held. Further, when a pulsevoltage of read voltage VR is applied to the selected word line afterthe sense line is precharged to the supply voltage level VDL, a signalvoltage corresponding to the information held in memory node N is readout at the sense line.

When for instance, information “1” is stored, the voltage of memory cellnode at VN(H) is a VN′ (H) higher than the threshold voltage VTR oftransistor QR due to the coupling capacitor Cc, so the transistor QRconducts (turns on), and the sensor line SL precharged to supply voltageVDL is discharged to ground potential VSS. On the other hand, when theinformation “0” has been stored, the voltage of a memory cell node atVN(L) becomes VN′ (L) lower than the threshold voltage VTR of transistorQR due to coupling capacitor Cc so that the transistor QR is held incutoff state, and the precharged sense line SL is held at supply voltagelevel VDL. As a result, the desired voltage is extracted externally,from the signal read from the sense line SL by way of the sequenceselected read circuit, and sets to read operation.

In the above description, the capacitive coupling 2-transistor cellshown in FIG. 5, was used in the hierarchical word structure shown inFIG. 7 and explained by focusing mainly on the each circuit of the subword driver. Further, the driving of the selected word line to threevoltage values was shown while adequately reducing the voltage appliedto the gate oxide film of each MOS transistor.

Among these, an example was shown for driving the main word line MWL bythe read control signal φr in FIG. 9, however the main word line MWL mayalso be driven by a read control circuit utilizing the decode signal axjand the write control signal φwb in FIG. 12. Further, in the capacitivecoupling 2-transistor cell shown in FIG. 5, the transistor QW utilizedthe tunnel effect however NMOS transistor operation is used so a normalNMOS transistor may be utilized at transistor QW.

In the three-transistor cell typified as shown in FIG. 4, when using thehierarchical word line structure shown in FIG. 7 with the memory cellcontrolling read operation of the three value word line voltage, themethods in FIG. 1, FIG. 9 and in FIG. 10 can be applied in order todrive the selected sub-word line to the three voltage values whileadequately reducing the voltage applied to the gate oxidation film ofeach MOS transistor in the circuit. Another structure of the sub-worddriver is explained next.

<Second Embodiment>

A typical circuit structure of a sub-word driver not having theoxide-stress relaxation transistor is shown in FIG. 13.

Compared to the circuit structure of the sub-word driver shown in FIG.1, in the sub-word driver 61 of FIG. 13 the PMOS transistor Mp2 and theNMOS transistor Mn5 have been removed. Other points of difference arethat the NMOS transistor Mn4 for selecting the read voltage has beeneliminated and the NMOS transistor Mn3 is jointly used, also the sourceof transistor Mn3 is connected to the common word line FXtn.Accordingly, main features are that the sub-word driver for driving theselected sub-word line to three voltage values, can be comprised of fourMOS transistors and that increases in the surface area of the circuitcan be restricted.

In this kind of circuit structure, the gate oxidation film thickness ofthe MOS transistors comprising the sub-word driver can be madesufficiently thick, and the electric field in the gate oxidation filmbetween the gate and source, and the gate and drain of the MOStransistors can be prevented from exceeding the maximum electric fieldEox max. Also, the preferred voltage settings in the capacitive coupling2-transistor DRAM cell shown in FIG. 6, can also be applied so thesub-word line voltage amplitude is near the supply voltage level VDLamplitude, and to prevent the electric field in the gate oxidation filmbetween the gate and source, and the gate and drain of the MOStransistors from exceeding the maximum electric field Eox max.

The circuit structure shown in FIG. 9 is utilized in the main worddriver MWD for driving the main word lines MWLbp, MWLbn, and MWLRtnconnected to the sub-word driver shown in FIG. 13.

On the other hand, the common main word driver FXD connectedrespectively to the common word lines FXtp, FXtn, and FXbn is shown inFIG. 14. The points of difference compared to the common word driver FXDshown in FIG. 10 is that a read voltage VR is input to the source of thePMOS transistor Mp1 in the inverter circuit NVL1 for driving the commonword line FXtn. The voltage amplitude of the common word line FXtnsignal therefore changes from standby voltage −VB to the read voltageVR.

The operation of the sub-word driver of FIG. 13 is shown in FIG. 15. Theoperation timing for the sub-word driver that generates the three valuevoltage levels is shown in the drawings in FIG. 15, and shows anoperation timing the same as in FIG. 8. Compared to the operation shownin FIG. 8, for the sub-word driver of FIG. 1, the point of differencehere is the operation when the common word line FXtn is selected.

The case when the main word line and the common word line are bothselected is explained. The selected main word driver MWD1 drives themain word line MWLRtn1 from standby voltage −VB in read operation, tothe supply voltage VDL. Also, the common word driver, drives the commonword line FX11 tn from standby voltage −VB to read voltage VR.Therefore, the transistor Mn3 conducts, the sub-word line SWL111 isselected, and the sub-word line SWL111 at standby voltage −VB, is drivento read voltage VR.

The case when the main word line is not selected, and the common wordline is selected is explained next. When the sub-word line SWL111 isselected, the sub-word line SWL211 for instance, sets to this state. Themain word driver MWD2 is held in non-select status, and the main wordlines MWL2 bn, MWLR2 tn are respectively held at supply voltage VDL andstandby voltage −VB. The common word driver on the other hand, drivesthe common word line FX11 tn from standby voltage −VB to read voltageVR. The transistor Mn3 is therefore cutoff, the transistor Mn1 conducts,and the sub-word line SWL211 is set to non-select status at standbyvoltage −VB.

The circuit structure in FIG. 13, showed a configuration where the mainword line MWLRtn is connected to the gate of transistor Mn3, and thecommon word line FXtn is connected to the source of the transistor Mn3.However, a circuit structure having the common word line FXtn connectedto the gate of transistor Mn3, and the main word line MWLRtn connectedto the source of transistor Mn3 may also be used. In this case, astructure is utilized for the main word driver shown in FIG. 9, where aread voltage VR is input to the source of transistor Mp1 in the levelshift circuit LSCL2 and the signal amplitude of the main word lineMWLRtn is shifted from standby voltage −VB to the read voltage VR.Further, for the circuit structure of the common word driver shown inFIG. 10, the signal amplitude of the common word line FXtn is shiftedfrom standby voltage −VB to supply voltage VDL.

<Third Embodiment>

Still another embodiment of the sub-word driver is shown in FIG. 16.

Unlike the sub-word driver shown in FIG. 1, this circuit ischaracterized by a simpler circuit structure since the NMOS transistorsMn3, Mn4 and main word line MWLR5 n as well as common word line FXtn areeliminated. This kind of circuit structure is also characterized in thatthe voltage potential on the common word line FXtp connected to thesource of PMOS transistor Mp1 is controlled according to the read orwrite operation.

The operation of the sub-word driver SWD of FIG. 16 is explainedaccording to FIG. 17. This figure shows the case when the sub-word lineSWL111 is selected, and the state is consecutively changed from standbystate to read operation. The main word driver MWD1 drives the main wordline MWL1 bp from write voltage VW to ground potential VSS, and the mainword line MWL1 bn is driven from supply voltage VDL to standby voltage−VB. In this state, first of all, when the read control signal φr atground potential VSS is driven to read status at supply voltage VDL, thecommon word driver FXD11 drives the common word lines FX11 tp, FX11 bnrespectively from ground potential VSS and supply voltage VDL, to readvoltage VR and standby voltage −VB. Therefore, the main word lines MWL1bp, MWL1 bn are respectively driven to ground potential VSS and standbyvoltage −VB, and the common word lines FX11 tp, FX11 bn are respectivelydriven to read voltage VR and standby voltage −VB, so that thetransistor Mp1 conducts, the sub-word driver SWD111 is selected, and thesub-word line SWL111 at standby voltage −VB is driven to read voltageVR.

Next, when the read control signal φr at supply voltage VDL is driven toground potential VSS and write status, the common sub-word driver FXD11drives the common word line FX11 tp from read voltage VR to writevoltage VW. The main word lines MWL1 bp, MWL1 bn therefore remain drivenrespectively to ground potential VSS and standby voltage −VB and areheld there. The common word lines FX11 tp, FX11 bn are respectivelydriven to the write voltage VW and standby voltage −VB, so that thetransistor Mp1 conducts, the sub-word driver SWD111 is selected, and thesub-word line SWL111 at read voltage VR is driven to the write voltageVW.

The main word drivers MWD respectively driving the main word linesMWLbp, MWLbn connecting to the sub-word driver shown in FIG. 16, and thecommon word driver FXD driving the common word lines FXtp, FXtn areexplained below.

First of all, the main word driver MWD in FIG. 18 is described. Asexplained previously in the operation shown in FIG. 17, a read controlcircuit is not required in the main word driver in order to executecontrol according to read and write operation in the common word driverof the third embodiment. The main word driver MWD is therefore comprisedof level shift circuits LSCH, LSCL to independently drive the main wordlines MWLbp, MWLbn. In other words, the decode signal axj is input tothe level shift circuits LSCH, LSCL, and their respective outputs usedfor the main word lines MWLbp, MWLbn. The decode signal axj is selectedon being driven to supply voltage level VDL, and the main word lineMWLbn is driven from write voltage VW to ground potential VSS, and themain word lines MWLbp is driven from supply voltage VDL to standbyvoltage −VB.

The common word driver FXD of FIG. 19 is described. The common worddriver FXD is comprised of the level shift circuits LSCHRW, LSCL toindependently drive the common word lines FXtp, FXbn, and the invertercircuits NV1, NV2. The read/write control circuit VRWCC is comprised ofthe level shift circuit LSCH described in the first embodiment and thevoltage switcher circuits VSW1, VSW2. An inverted signal ajb received byway of the inverter circuit NV1 from the decode signal aj, is input tothe level shift circuit LSCH, and the output of the level shift circuitLSCH used as the read control signal ΦR. Therefore, the read controlsignal φr with an amplitude from ground potential VSS to supply voltageVDL, becomes the read control signal ΦR, with a voltage amplitude fromground potential VSS to write voltage VW. The voltage switcher circuitVSW1 is comprised of PMOS transistor Mp1 and NMOS transistor Mn1. Theread control signal ΦR is connected to the gate of the transistors Mp1and Mn1, and a write voltage VW is input to the source of transistorMp1, and a read voltage VR is input to the source of the transistor Mn1.A read/write voltage VRW is applied to the drains of transistors Mp1 andMn1.

The voltage switcher circuit VSW2 is comprised of PMOS transistor Mp1and NMOS transistor Mn1. The read control signal φr is connected to thegates of the transistors Mp1 and Mn1, a supply voltage VDL is input tothe source of transistor Mp1, and a read voltage VR is input to thesource of the transistor Mn1. A cutoff voltage VRDL is applied to thedrain of the transistors Mp1 and Mn1. The level shift circuit LSCHRW isdifferent from the level shift circuit LSCH described in the firstembodiment, in the point that a read/write voltage VRW is input to thesource of the PMOS transistors Mp1, Mp2, and that a cutoff voltage VRDLis input to the NMOS transistor Mn2. In the level shift circuit LSCHRWof this kind of structure, a decode signal aj is connected to the sourceof transistor Mn2 and the gate of transistor Mn1, and the drains oftransistor Mn1 and Mp1 as well as the gate of transistor Mp2 are usedfor the common word line FXtp. An inverted signal ajb received by way ofthe inverter circuit NV2 from the decode signal, is input to the levelshift circuit LSCL and that output used for the common word line FXbn.

In the common word driver FXD configured as described above, the decodesignal aj is selected by reaching ground potential VSS. First of all,when the read control signal φr at ground potential VSS is driven toread status at supply voltage VDL, this signal is input to the voltageswitcher circuit VSW2 so that the transistor Mn1 conducts, and thecutoff voltage VRDL at supply voltage VDL is driven to the read voltageVR. Also, the read control signal ΦR is set to write voltage VW so thatthe read/write voltage VRW is from write voltage VW to read voltage VR.Accordingly, in the level shift circuit LSCHRW, a ground potential VSSdecode signal aj, a read/write voltage VRW of read voltage VR and acutoff voltage VRDL are input so that the transistors Mn1, Mp2 reach acutoff state, the transistors Mn2, Mp1 conduct, and the common word lineFXtp is driven from ground potential VSS to read voltage VR. Also, asupply voltage VDL is input in the level shift circuit LSCL, so that thecommon word line FXbn is driven from is driven from supply voltage VDLto standby voltage −VB. Next, when the read control signal φr at supplyvoltage VDL is driven to ground potential VSS reaching write operation,this signal is input to the voltage switcher circuit VSW2 so that thetransistor Mp1 conducts, and the cutoff voltage VRDL is driven from readvoltage VR to the supply voltage VDL. Also, the read control signal ΦRchanges from write voltage VW to ground potential VSS so that theread/write voltage VRW is driven from read voltage VR to write voltageVW. Therefore, in the level shift circuit LSCHRW, a ground potential VSSdecode signal aj, a read/write voltage VRW of the write voltage VW and acutoff voltage VRDL are input so that the transistors Mn1, Mp2 reach acutoff state, the transistors Mn2, Mp1 conduct, and the common word lineFXtp is driven from read voltage VR to write voltage VW. Also aninverted decode signal ajb of the supply voltage VDL is input in thelevel shift circuit LSCL, and the common word line FXbn is held atstandby voltage −VB.

The operation of the level shift circuit LSCHRW on the other hand, innon-select state is different from the level shift circuit LSCH, inorder to switch the write voltage VRW. In other words, in the non-selectstate, the decode signal aj of supply voltage VDL is input, andtransistor Mn1 conducts, and the common word line FXtp is driven toground potential VSS. Here, in the write operation and in the standbystate, the read/write voltage of write voltage VW as well as the cutoffvoltage VRDL of the supply voltage VDL are input so that the transistorMp2 conducts, and the transistor Mp2 reaches the cutoff state. Then, aread/write voltage VRW of write voltage VW is input to the drain of thetransistor Mn2 so that the transistor Mn2 reaches the cutoff state, andthe DC current by way of the transistor Mp2 is cutoff. Further, theread/write voltage VRW of read voltage VR as well as the cutoff voltageVRDL, are input in the read operation so that the transistor Mp2conducts, and the transistor Mp1 reaches the cutoff state. Then, theread/write voltage VRW of read voltage VR is input to the drain of thetransistor Mn2 so that the transistor Mn2 reaches cutoff state, and theDC current is cutoff by way of the transistor Mp2.

As previously explained, the common word driver of this embodiment shownin FIG. 19, is characterized in driving the common word line to threevoltage levels. More specifically, a read/write control circuit VRWCC4controls the voltage according to the read/write operation. This worddriver is further characterized by switching to a cutoff voltage VRDLaccording to the voltage selection, in order to prevent DC current fromflowing in the level shift circuit LSCHRW.

In the preferred voltage settings for the capacitive coupling2-transistor DRAM shown in FIG. 6, the threshold voltage of transistorMn2 is sufficiently larger than the read voltage VR so that when thedrive performance of transistor Mn2 is sufficiently large, a fixedcutoff voltage VRDL may be used in the read voltage VR. Further, in thevoltage switcher circuit VSW1, the read/write voltage VRW is driven bytwo different positive voltages by way of the transistors Mp1 andtransistor Mn1 having different well structures, so that the occurrenceof a latchup is prevented when the power is turned on, and a writevoltage VW definitely higher than the supply voltage VDL can begenerated.

A summary of the sub-word driver shown in FIG. 16 is next explained. Byutilizing the common word driver FXD shown in FIG. 19, the sub-worddriver shown in FIG. 16 for outputting three voltage values can becomprised by five MOS transistors. Also the circuit structure of thisportion is simple since it is comprised by two main word lines and twocommon word lines, and the surface area can therefore be kept small. Byutilizing the circuit shown in this embodiment with the preferredvoltage settings for the capacitive coupling 2-transistor DRAM shown inFIG. 6, the breakdown voltage problem in the gate oxidation film can beresolved, as is easily understood from the description of the firstembodiment. Alternatively, the method utilizing n+si gates in the PMOStransistors Mp1, Mp2 described in the first embodiment, or the methodfor suitably amplifying the level-shifted main word line signal andcommon word line signal, can also be applied to the circuits shown inthis embodiment. Also, the application of a fixed level voltage to thegates of the oxide-stress relaxation transistors Mp2, Mn5, is notlimited to the one method the same as the first embodiment, but mayinstead employ a pulse having a suitable voltage amplitude. Further,when the electric field in the gate oxidation film between the gate andsource, and between the gate and drain of the MOS transistors, asrelated in the second embodiment, does not exceed the maximum electricfield Eox max, then the circuit structure need not include theoxide-stress relaxation transistors Mp2, Mn5. Further by sharing(consolidating) the main word lines MWLbp, MWLbn shown in FIG. 16 whenthe electric field in the gate oxidation film is sufficiently small, thesub-word driver can be driven by one main word line and an increase inthe surface area of the circuit thus be prevented. Still further, in thecommon sub-word driver shown in FIG. 19, instead of the method forcontrolling the common word line by using the read control signal φr asdescribed in the first embodiment, a write control signal φwb can beutilized for controlling the common word lines.

<Fourth Embodiment>

Yet another structure of the sub-word driver as shown in FIG. 20 isdescribed.

The sub-word driver SWD of this embodiment differs from the sub-worddriver shown in FIG. 16, in being characterized that the sourceelectrode of the NMOS transistor Mn2 is connected to the common wordline FXtn, without being connected to standby voltage −VB. The main wordline signal handles the PMOS transistor Mp1 and the NMOS transistor Mn1separately, and connects the main word line MWLbp to the gate of PMOStransistor Mp1, and the main word line MWLbn to the gate of the NMOStransistor Mn1. Also, the main word line MWLtn is connected to the gateof the NMOS transistor Mn2. The common word line also handles the PMOStransistor Mp1 and the NMOS transistor Mn2 separately, and the commonword line FXtp is connected to the source of the PMOS transistor, andthe common word line FXtn is connected to the source of the NMOStransistor Mn2. The source of the NMOS transistor Mn1 is connected tothe standby voltage −VB. The PMOS transistor Mp2 and the NMOS transistorMn5 are oxide-stress relaxation MOS transistors, and a fixed voltage isapplied to the gate electrode. An example respectively applying bothground potential VSS and supply voltage VDL is shown in FIG. 20. Thesub-word line SWL is connected to the drains of the transistor Mp2, Mn2as well as Mn5. By utilizing this type of circuit structure, the problemof breakdown voltage in the gate oxidation film can be resolved, and asub-word driver comprised of three main word lines and two common wordlines as well as five MOS transistors. This circuit configuration isalso characterized in that the voltage potential on the common word lineFxtp connected to the source of the PMOS transistor Mp1 is controlledaccording to the read operation and write operation, to generate wordline voltages of three values.

A circuit configured as described here can adequately achieve theobjects of the invention without utilizing the transistors Mp2 and Mn5.

The operation of the sub-word driver SWD of FIG. 20 is next describedaccording to FIG. 21. The sub-word line SWL111 in this figure is shownwhen in the selected state, and changes from standby state to readoperation and write operation occur consecutively. The point differingfrom the first embodiment is that the selected word driver such as themain word driver MWD1, in either the read operation or write operation,the main word lines MWLbp, MWLbn and MWLtn are respectively driven toground level VSS, standby voltage −VB, and supply voltage VDL. The mainword driver MWD1 respectively drives the main word line MWL1 bp fromwrite voltage VW to ground potential VSS, the main word line MWL1 bnfrom the supply voltage VDL to standby voltage −VB, and the main wordline MWL1 tn from standby voltage −VB to the supply voltage VDL. In thisstate, first of all, when the read control signal φr at ground potentialVSS is driven to the supply voltage level VDL to reach read operation,the common word driver FXD11 drives the common word lines FX11 tp, FX11tn, respectively from ground potential VSS and standby voltage −VB tothe read voltage VR. The main word lines MWLbp, MWLbn and MWLtn1 arerespectively driven to ground potential VSS, standby voltage −VB and tosupply voltage VDL, and the common word lines FX11 tp, FX11 tn arerespectively driven to the read voltage VR, so that the transistor Mn1sets to cutoff state, the transistors Mp1, Mp2 conduct, the sub-worddriver SWD111 is selected, and the sub-word line SWL111 is driven fromstandby voltage −VB to the read voltage −Vr.

Next, when the read control signal φr at supply voltage VDL is driven tothe ground potential VSS, the common word driver FXD11 drives the commonword line FX11 tp from read voltage VR to the write voltage VW, and thecommon word line FX11 tn to the supply voltage VDL. The main word linesMWLbp, MWLbn and MWLtn1 are therefore respectively driven to groundpotential VSS, standby voltage −VB and to supply voltage VDL and held inthose states, and the common word lines FX11 tp, FX11 tn arerespectively driven to the write voltage VW and the supply voltage VDLso that the Mn1 and Mn2 reach the cutoff state, the transistor Mp1conducts and the sub-word driver SWD111 is selected, and the sub-wordline SWL111 is driven from read voltage VR to the write voltage VW.

The main word lines MWLbp and MWLbn connected to the sub-word drivershown in FIG. 20, are respectively driven by the main word driver MWD,and the common word driver lines FXtp, FXtn are driven by the commonword driver FXD as shown below.

The operation of the main word driver of FIG. 22 is explained next. Justthe same as in the operation described for FIG. 21, a read/write controlcircuit is not required in the common word driver in the fourthembodiment for executing control according to the read operation andwrite operation. The main word driver MWD is therefore comprised oflevel shift circuits to independently drive the main word lines MWLbp,MWLbn, and an inverter circuit NVL to drive the main word line MWLtn. Inother words, the decode signal axj is input to the level shift circuitLSCH, LSCL, and the respective outputs used for the main word linesMWLbp and MWLbn. The main word line MWLbn is connected to the invertercircuit NVL, and that output is used for the main word line MWLtn. Thedecode signal axj is selected by reaching the supply voltage level VDL,the main word line MWLbp at the write voltage VR is driven to groundpotential VSS, the main word line MWLbn at the supply voltage VDL isdriven to the standby voltage −VB, and the main word line MWLtn at thestandby voltage −VB is driven to the supply voltage VDL.

The operation of the common word driver shown in FIG. 23 is describednext. The common word driver FXD is comprised of the level shiftcircuits LSCHRW and LSCLR to separately drive the common word linesFXtp, FXtn, and the read/write voltage control circuit VRWCC5 andinverter control circuit NV1. The point differing from the common worddriver of the third embodiment shown in FIG. 19 is that the invertercircuit NVL2 is eliminated since the polarity of common word line FXnbis the same as the common word line FXnt reversed polarity, and thedecode signal aj is input as is, to the level shift circuit LSCLR. Also,the level shift circuit LSCLR, unlike the level shift circuit LSCL ofthe first embodiment, is input at the source of transistor Mp1 with acutoff voltage VRDL. A common word driver FXD of this type is thereforeselected when the decode signal aj reaches ground potential VSS, and thecommon word line FXnt at standby voltage −VB is driven to the readvoltage VR for read operation, and to the supply voltage VDL for writeoperation. The operation of the common word line FXtp is the same as thecommon word driver of the third embodiment shown in FIG. 19.

Therefore, the same as in the common word driver of the third embodimentshown in FIG. 19, the common word driver of this embodiment shown inFIG. 23, is characterized by driving the common word line FXtp to threevoltage levels while switching the cutoff voltage VRDL according to thechanged voltages in order to prevent DC current from flowing in thelevel shift circuit LSCHRW. Also, the sub-word driver is characterizedin that in order to control the transistor Mn2, the common word lineFXtn is driven to three word lines: a standby voltage −VB, a readvoltage VR and a supply voltage VDL. In the preferred voltage settingsfor the capacitive coupling 2-transistor DRAM shown in FIG. 6, when theread voltage VR is sufficiently larger than the threshold voltage oftransistor Mn2, and the drive performance of transistor Mn2 issufficiently large, the gate voltage for transistor Mn2 in the levelshift circuit LSCHRW, may be fixed at the read voltage VR. Also, in thevoltage switcher circuit VSW1, the read/write voltage VRW is driven bytwo different positive voltages, by way of the transistors Mp1 andtransistor Mn1 having different well structures, so that the occurrenceof a latchup is prevented when power is turned on, and a write voltageVW definitely higher than the supply voltage VDL can be generated.

A summary of the sub-word driver shown in FIG. 20 is next explained. Byutilizing the common word driver FXD shown in FIG. 23, the sub-worddriver shown in FIG. 20 for outputting three voltage values can becomprised by five MOS transistors. Also, the circuit structure of thisportion is simple since it is comprised by three main word lines and twocommon word lines, and the surface area can therefore be kept small.

By utilizing the circuit shown in this embodiment with the preferredvoltage settings for the capacitive coupling 2-transistor DRAM shown inFIG. 6, the breakdown voltage problem in the gate oxidation film can beresolved, as is easily understood from the description of the firstembodiment. Alternatively, the method utilizing n+si gates in the PMOStransistors Mp1, Mp2 described in the first embodiment, or the methodfor suitably amplifying the level-shifted main word line signal andcommon word line signal, can also be applied to the circuits shown inthis embodiment. Also, the application of a fixed level voltage to thegates of the oxide-stress relaxation transistors Mp2, Mn5, is notlimited to the one method the same as the first embodiment, but mayinstead employ a pulse having a suitable voltage amplitude. Further,when the electric field in the gate oxidation film between the gate andsource, and between the gate and drain of the MOS transistors, asrelated in the second embodiment, does not exceed the maximum electricfield Eox max, then the circuit structure need not include theoxide-stress relaxation transistors Mp2, Mn5. Further by sharing(consolidating) the main word lines MWLbp, MWLbn shown in FIG. 20 whenthe electric field in the gate oxidation film is sufficiently small, thesub-word driver can be driven by two main word lines and two common wordlines and an increase in the surface area of the circuit thus beprevented. Still further, in the common sub-word driver shown in FIG.23, instead of the method for controlling the common word lines by usingthe read control signal φr as described in the first embodiment, a writecontrol signal φwb can be utilized for controlling the common wordlines.

Various embodiments conforming to this invention were described abovehowever this invention is not limited to these structures and the sameeffect can be also obtained by different variations and applications.For instance, the use of a hierarchical word line structure in thisinvention was explained, however this invention is also applicable tothe case of using a conventional word line structure with the worddriver controlled directly by a low decoder. This invention was alsodescribed with the hierarchical word line structure shown in FIG. 7 withthe capacitive coupling 2-transistor cell of FIG. 5, and was furtherdescribed using a memory cell with a hierarchical word line structure tocontrol the read/write operation at three word line voltages, astypified in the three-transistor cell shown in FIG. 4, however variousadaptations and applications of this invention are possible for drivingthe selected sub-word lines to voltages of three values, whilesufficiently reducing the voltage applied to the gate oxidation film ofthe MOS transistors in each circuit.

The above embodiments were also explained for the case when thethreshold voltage VTR of the read transistor QR was lower than thethreshold voltage VTW of the write transistor QW, clearly however, thesame reasoning is also valid if the relation of the read transistor QRthreshold voltage VTR and the transistor QW threshold voltage VTW areinterchanged. In such a case, the data lines for read and for write canbe isolated and controlled as needed, and the memory cell read operationcan be performed by setting the read voltage to VW and the write voltageto VR. At this time, the main word driver, the common word driver andthe sub-word driver, may be configured to drive the sub-word line toread voltage and to the write voltage, while appropriately controllingthe read/write control circuit shown in the various embodiments.

The explanation for the above embodiments also described configuring amemory cells using NMOS transistors however clearly, same reasoning isalso valid if the memory cells are comprised using PMOS transistors. Insuch a case, along with interchanging the PMOS and NMOS transistors inthe sub-word driver, the power supply voltage relation such as for thesupply voltage, main word lines and also the common word lines may alsobe reversed,and the non-select sub-word line voltage set higher than thehigh level data line, and the select sub-word line voltage set lowerthan the data line low level.

The invention as described above therefore reduces the problem ofbreakdown voltage in the MOS transistors and provides a DRAM forcontrolling read and write at three values of word line voltages.

1. A semiconductor device comprising: a plurality of memory cellscoupled to a plurality of word lines and a plurality of data lines; anda plurality of word drivers controlling voltages (SWL) applied to saidplurality of word lines, wherein a voltage applied to one of saidplurality of word lines for a read operation is lower than a voltageapplied to one of said plurality of word lines for a write operation,wherein said voltage applied to one of said plurality of word lines forsaid read operation is higher than a voltage applied to one of saidplurality of word lines which are not selected for either said readoperation or said write operation, and wherein said plurality of memorycells each comprises at least a transistor having a gate coupled to saidone of said plurality of word lines, and having a source/drain pathbetween one of said plurality of data lines and a node supplied with apredetermined voltage.
 2. The semiconductor device according to claim 1,wherein a voltage applied to said plurality of word lines during aprecharge operation is lower than a ground voltage.
 3. The semiconductordevice according to claim 1, wherein said voltage applied to one of saidplurality of word lines for said read operation is lower than a voltageof one selected data line of said plurality of data lines.
 4. Thesemiconductor device according to claim 3, wherein said voltage appliedto one of said plurality of word lines for said write operation ishigher than said voltage of one selected data line of said plurality ofdata lines.
 5. The semiconductor device according to claim 4, whereinsaid plurality of memory cells are dynamic memory cells.
 6. Asemiconductor device comprising: a plurality of memory cells coupled toa plurality of word lines and a plurality of data lines; and a pluralityof word drivers controlling voltage applied to said plurality of wordlines; wherein a first voltage applied to one of said plurality wordlines for a read operation is lower than a second voltage applied to oneof said plurality of word lines for a write operation, wherein saidfirst voltage is higher than a third voltage applied to one of saidplurality of word lines which are not selected for either said readoperation or write operation, wherein each of said plurality of worddrivers includes a first MOS transistor having a source and a drainwhich are coupled between a corresponding one of said plurality of wordlines and said first voltage, a second MOS transistor having a sourceand a drain which are coupled between a corresponding one of saidplurality of word lines and said second voltage, and a third MOStransistor having a source and a drain which are coupled between acorresponding one of said plurality of word lines and said thirdvoltage.
 7. The semiconductor device according to claim 6, wherein saidplurality of memory cells each comprises at least a transistor having agate coupled to said one of said plurality of word lines, and having asource/drain path between one of said plurality of data lines and a nodesupplied with a predetermined voltage.
 8. The semiconductor deviceaccording to claim 6 wherein a voltage applied to said plurality of wordlines during a precharge operation is lower than a ground voltage. 9.The semiconductor device according to claim 8, wherein said voltageapplied to one of said plurality of word lines for said read operationis lower than a voltage of one selected data line of said plurality ofdata lines.
 10. The semiconductor device according to claim 9, whereinsaid voltage applied to one of said plurality of word lines for saidwrite operation is higher than said voltage of one selected data line ofsaid plurality of data lines.
 11. The semiconductor device according toclaim 9, wherein said plurality of memory cells are dynamic memorycells.